One way to increase power efficiency in a power converter is through the use of synchronous rectifiers to replace conventional freewheeling diodes. A feature of DC-DC power converters having synchronous rectification is that current is enabled to flow not only to the output terminals through the synchronous rectifier, but also in a reverse direction from the output terminals back into the converter.
FIG. 1 is a schematic diagram of a prior art power converter 10 described in U.S. Pat. No. 6,181,578, issued Jan. 30, 2001, entitled “Synchronous Rectifier Drive Mechanism for Resonant Reset Forward Converters”, commonly assigned and incorporated by reference herein. As seen in FIG. 1, power converter 10 is a resonant reset forward converter including a gate drive mechanism for controlling the conduction periods of a free-wheeling synchronous rectifier on the secondary side of the converter. A power switch 16 is connected in series with the primary winding 13 of a transformer 12 at node 102. The primary winding 13 of the transformer 12 and the power switch 16 are connected across an input DC voltage source 11. The power switch 16 is alternately switched between an on period and an off period in response to a pulse width modulated (PWM) signal applied to the control gate of the power switch 16 by a PWM generator (not shown). The alternating states of the power switch 16 causes an AC voltage to be generated across the secondary winding 14 of the transformer 12. The signal provided by the PWM is generated in response to a feedback signal from a feedback loop (not shown) which is coupled across the output terminals Vout1 and Vout2 of the converter 10 and is responsive to the output voltage of the converter 10.
On the secondary side of the forward converter 10, the secondary winding 14 of the transformer 12 is coupled to a positive voltage rail at node 201 and a negative voltage rail at node 202. A forward rectifier 112 is coupled between the secondary winding 14 of the transformer 12 and the secondary side ground. The forward rectifier 112 is typically a MOSFET having a source, a drain, and a gate. The gate of the forward rectifier 112 is coupled to node 201. The drain of the forward rectifier 112 is coupled to node 202. The source of the forward rectifier 112 is coupled to the secondary side ground. The forward rectifier 112 provides a forward conduction path between the secondary winding 14 (at node 202) and the second output terminal of the converter 10 (Vout2).
A free-wheeling synchronous rectifier 116 is coupled between node 201 and the source of the forward rectifier 112. In an exemplary embodiment, the free-wheeling synchronous rectifier 116 is a MOSFET having a source, a drain, and a gate. The drain of the free-wheeling synchronous rectifier 116 is coupled to node 201. The source of the free-wheeling synchronous rectifier 116 is coupled to a shoot-through inductor 118. The control gate of the free-wheeling synchronous rectifier 116 is connected to a secondary side diode 115. The secondary side diode 115 has an anode connected to the drain of the forward rectifier 112. The cathode of the secondary side diode 115 is connected to the gate of the free-wheeling synchronous rectifier 116. The free-wheeling synchronous rectifier 116 is operative to provide a current path between the positive voltage rail and the output terminal VOUT2 when turned on by application of a suitable charge to its gate.
A quench FET 114 is coupled between the gate of the free-wheeling synchronous rectifier 116 and the secondary side ground and is operative to rapidly turn-off of the free-wheeling synchronous rectifier 116 at the beginning of the forward power cycle of the converter 10. In one exemplary embodiment, the drain of the quench FET 114 is coupled to the gate of the free-wheeling synchronous rectifier 116 and the cathode of the secondary side diode 115 at node 117. The source of the quench FET 114 is connected to the secondary side ground. The gate of the quench FET 114 is connected to the positive voltage rail at node 201. With this configuration, the free-wheeling synchronous rectifier 116 is maintained in a high impedance state, i.e., a non-conducting state, when the power switch 16 is turned on and the forward converter 10 is in the forward power cycle.
The shoot-through inductor 118: (1) initiates the discharging of the inherent drain to source capacitance, Cds, of the free-wheeling rectifier 116 at the beginning of each forward power cycle; and (2) reduces the gate to source voltage, Vgs, across free-wheeling rectifier 116 during this transition, thereby causing the free-wheeling rectifier 116 to rapidly turn off. The quench FET 114 finishes the discharging of the free-wheeling rectifier 116 by shunting the charge present on the gate of the free-wheeling rectifier 116 to ground at the beginning of the forward power cycle. Discharging the gate of the free-wheeling rectifier 116 at the beginning of the forward power cycle prevents a large simultaneous current flow through the forward rectifier 112 and the free-wheeling rectifier 116.
An output filter 100, consisting of a filter inductor 120 connected in series to a filter capacitor 122, is coupled across the output terminals Vout1 and Vout2 of the converter 10. The output filter 100 filters out any high frequency components of the ripple current present along the positive voltage rail and provides a substantially ripple free, substantially constant DC output voltage Vo across the output terminals of the forward converter 10. Also shown coupled across the output terminals of the forward converter is an exemplary load, RLOAD.
Consequently, the gate drive mechanism shown in FIG. 1 is operative to turn on the free-wheeling synchronous rectifier at the beginning of the forward power cycle, maintain the free-wheeling rectifier in the on state during the transformer core reset and dead periods, and provide for rapid discharging of the freewheeling rectifier at the beginning of a subsequent forward power cycle.
A disadvantage of the forward converter shown in FIG. 1 is that the gate charge of the free-wheeling synchronous rectifier cannot be fully discharged quickly when the PWM signal turns off or has missing cycles due to a failure condition. As a result, a large negative current, which reflects to the primary main power switch via the power transformer, is enabled to flow in a reverse direction from the output terminals back into the converter through the free-wheeling synchronous rectifier.
The bi-directional current flowing capability of the synchronous rectifier may pose a serious problem, i.e., a large negative current flow could result in the destruction of the freewheeling synchronous rectifier or primary power MOSFET when such rectifiers are used in paralleled power converters having a shared output bus, unless an ORing diode is used to protect each power converter. In other words, although the paralleling of power converters provides a way for two or more individual, small, high density power converter modules to supply the higher power required by current generation loads and/or to provide redundancy, this topology also enables the current share bus to couple back into the converter an uncontrolled amount of current when the free-wheeling synchronous rectifier is conducting.
FIG. 2 is a block diagram of a system of parallel converters (also referred to herein as “power modules”). For the paralleled converter configuration shown in FIG. 2, power is supplied to a common output voltage bus and thereby to a load. As shown in FIG. 2, power module 1, power module 2, . . . power module N are each coupled to a single power output port 25 for supplying power to a load. An exemplary load 26 is shown coupled to output port 25 of system 20. In a preferred embodiment, power is supplied to power modules 1 through N at a single power input port 22. It will be recognized by those skilled in the art that it is not necessary that power be supplied to power modules 1 through N at a single power input port. Rather, each power module may receive power from a separate power source such as separate AC-DC converters (not shown). Each power module in system 20 has a parallel pin 24 and, as shown in FIG. 2, the parallel pins 24 of power modules 1 through N are all coupled to each other via a bus 28.
Although any large negative current flow may pose serious problems, an advantage of the bi-directional current flowing capability of the synchronous rectifier in a power converter is that, by enabling a small negative current flow, the transient response stepping from full load to zero load is increased because the negative current aids in the discharge of the output voltage.
Conventional circuits for controlling the free-wheeling synchronous rectifier to prevent negative current flow are dependent on one or more of the following: the timing, current sense signals, voltage sense signal, the current share system, and the operation of the forward synchronous rectifier.
A need therefore exists for providing a circuit and method for controlling the free-wheeling synchronous rectifier in a power converter for preventing any large negative current flow during any fault condition and to do so in a circuit and using a corresponding method that is independent of timing, current sense and voltage sense signals, the current share system, and the operation of the forward synchronous rectifier, but is dependent solely on a PWM gate drive output. There is a particular need to provide such a circuit and corresponding method for power converters in a system having paralleled power converters.